Elevators comprising a traction sheave arranged to operate at least one, typically more, ropes to which an elevator car and a counterweight is attached are commonly used. The friction between ropes and traction sheave is critical in elevators. In normal operation conditions the friction between ropes and traction sheave is large enough so that when the motor operates the traction sheave the elevator moves up or down. The friction force is determined by a plurality of different factors, for example, the shape and texture of rope and traction sheave and weight of the elevator car and counterweight. Parameters of these factors are decided for various reasons, for example, energy efficiency and security requirements.
When the friction between the ropes and the traction sheave is high enough the ropes in the traction sheave do not slip even if the elevator car or the counterweight has stalled. Stalling can be caused, for example, by running failure where the elevator car or counterweight gets stuck in the elevator shaft. If the elevator car is going upwards and the downward going counterweight gets stuck there will be slack in the rope between the traction sheave and counterweight. The slack between the traction sheave and the counterweight, or the elevator car if the movement is in opposite direction, can cause inconveniences or even security risk. A further example of stalling is a situation wherein the counterweight, or an elevator car, is at the buffer but the hoisting machine is still operating.
Conventionally this problem has been solved by designing the system so that the friction is large enough to operate in normal conditions and in case of stalling it is not too high so that the rope will slip. This kind of designing sets limitation to the overall design of the elevator system and the additional mechanical movement causes inefficiency and unnecessary wear.
In an improved method the stalling situation is detected by monitoring the current required by the hoisting machine. For example, if the counterweight has stalled the gravity caused by the counterweight does not help in hoisting of the elevator car. Thus, the hoisting machine needs more electric current in order to maintain the upward movement of the elevator car. The stalling situation is detected if the current required by the hoisting machine exceeds a predetermined threshold value.
The drawback of the current threshold implementation is that the current does not exceed the threshold in all conditions. The current threshold must be above the highest allowable current and it is possible that it is not exceeded always when stalling has occurred.